1. Field of the Invention
The present invention relates to a strained silicon structure, and more particularly to a strained silicon structure having different channel strain in the high density region and in the low density region.
2. Description of the Prior Art
The performance of Si semiconductor elements, MOSFETs in particular, has increased year after year with the advance of large-scale integrated circuits (LSI). Recently, however, it has appeared that the miniaturization attained by lithographic technology has reached its limit, and that carrier mobility has almost reached theoretical mobility in Si. To attain higher performance of the semiconductor device, attempts have been made to use a strained silicon (Si) layer grown epitaxially on a Si wafer with a silicon germanium (SiGe) layer in a recess in the substrate.
Generally, the wafer can be separated into a high density region and a low density region. The high density region has higher density of devices, and the low density region has lower density of devices. The recesses for the SiGe layer in the high density region and the low density region are formed by using the same etching step. Because the device density is different in the high density region and the low density region, the recesses in the high density region and the low density region may have different shapes. After the SiGe layer is grown in the recesses with different shapes, the stress provided by the SiGe layer will be different between the high density region and the low density region. As a result, the strained Si channels in the high density region and the low density region may become different and deteriorate the performance of the entire device.